Generally, as disclosed in, for example, JP-A 54-12927, a conventional hydraulic circuit for a hydraulic machine comprises a hydraulic pump, an actuator, for example, a hydraulic motor for driving a load (or an inertia body) by hydraulic fluid from the hydraulic pump, a valve apparatus, for example, a directional control valve for controlling flow of hydraulic fluid supplied to the hydraulic motor from the hydraulic pump to control driving of the hydraulic motor, and a counter balance valve arranged between the directional control valve and the hydraulic motor, for preventing a load in the gravitational direction from causing the hydraulic motor to run away.
In the hydraulic circuit constructed as above, when, for example, the directional control valve is neutral, the hydraulic fluid from the hydraulic pump is not supplied to the hydraulic motor. Further, the hydraulic motor attempts to rotate by the load in the gravitational direction. Since, however, main lines through which return fluid from the hydraulic motor is introduced into a tank are shut off by the counter balance valve, it is impossible to discharge the return fluid. Accordingly, the hydraulic motor is maintained stationary.
When the directional control valve is switched to one direction from the neutral, the hydraulic fluid from the hydraulic pump is supplied to the hydraulic motor through the main line. The counter balance valve is also switched to one direction by the actuator driving pressure generated in the main line for driving the load, that is, a hydraulic-motor driving pressure. Thus, the main line on the discharge side, which has been shut off by the counter balance valve, is brought into communication with the tank. Return fluid discharged from the hydraulic motor is introduced into the tank. The hydraulic motor drives the load in a direction corresponding to the switched direction of the directional control valve, for example, in a direction against the force of gravity.
When the directional control valve is switched to the opposite direction, the hydraulic fluid from the hydraulic pump is supplied to the hydraulic motor through the main line. The counter balance valve is also switched to the opposite direction by the hydraulic-motor driving pressure generated in the main line. Thus, the main line on the discharge side is brought into communication with the tank, and the return fluid from the hydraulic motor is introduced into the tank, so that the hydraulic motor drives the load in the gravitational direction. In this case, if the quantity of the return fluid discharged into the discharge-side main line from the hydraulic motor increases more than the quantity of the hydraulic fluid supplied through the supply-side main line, the hydraulic-motor driving pressure within the supply-side main line decreases so that the counter balance valve is moved toward the neutral. Thus, the discharge-side main line is restricted, whereby the quantity of the return fluid discharged from the hydraulic motor is limited.
In the manner described above, when the load is driven in the gravitational direction, the counter balance valve operates in such a manner that hydraulic-motor driving pressure corresponding to a spring of the counter balance valve is generated in the supply-side main line, to restrict the discharge-side main line thereby limiting the return fluid. For this reason, rotation of the hydraulic motor is prevented from being brought to a valve higher than rotation corresponding to the hydraulic fluid supplied to the hydraulic motor through the supply-side main line, making it possible to prevent running-away of the hydraulic motor in the load direction due to the force of gravity. This is applicable also to a case where another actuator, upon which the inertia force acts, is arranged in substitution for the hydraulic motor.
Additionally, in the above-described conventional hydraulic circuit, the counter balance valve is provided in the main line through which the hydraulic pump communicates with the hydraulic motor, and a large quantity of hydraulic fluid is controlled by the counter balance valve. For this reason, the hydraulic circuit has an energy loss due to pressure loss. Further, an attempt to reduce the pressure loss leads to an increase in size of the counter balance valve, resulting in an increase in the manufacturing cost.
Furthermore, control of the large quantity of hydraulic fluid results in an increase in flow force due to hydraulic fluid acting upon the counter balance valve. It is difficult to design component parts to cope with this flow force. Thus, it is difficult to obtain stable performance.
Moreover, there is disclosed in the specification of U.S. Pat. No. 4,425,759 a valve apparatus which adopts means other than a counter balance valve to provide a function equivalent to the counter balance valve. The arrangement of the valve apparatus is such that two main lines are provided which are connected to a hydraulic actuator, check valves are provided respectively in the main lines for permitting hydraulic fluid to flow only toward the actuator, a pressure limiting valve of hydraulic-pressure control type is provided in a branch main line connected to the actuator in parallel with each check valve, and the pressure limiting valve has an output port which is connected to a return line communicating with a tank. With such arrangement, pressure in the other main line is introduced into a control section of the pressure limiting valve, to vary setting pressure in accordance with pressure within the main line.
In the valve apparatus, by switching-over of the directional control valve, the hydraulic fluid from the hydraulic pump is supplied to the actuator from one main line through the check valve, while hydraulic fluid discharged into the other main line from the actuator is discharged to the tank through the pressure limiting valve. At this time, if the quantity of the return fluid discharged into the discharge-side main line from the actuator increases more than that supplied through the supply-side main line, the actuator driving pressure within the supply-side main line decreases to increase the setting pressure of the pressure limiting valve. Thus, the discharge-side main line is restricted, whereby the quantity of the return fluid discharged from the hydraulic motor is limited.
However, the valve apparatus is the same as the counter balance valve in that the check valves and the pressure limiting valve are arranged in the main lines. Accordingly, problems arise which are similar to those described above in connection with the counter balance valve.
On the other hand, the specification of U.S. Pat. No. 4,535,809 discloses a valve apparatus which controls flow of hydraulic fluid supplied to a hydraulic actuator to control driving thereof. The valve apparatus comprises a main valve of seat type and a pilot circuit provided with a pilot valve for controlling an opening degree of the main valve. In this valve apparatus, the main valve is composed of an inlet port and an outlet port, a valve element of seat type, a variable restrictor having an opening degree varied depending upon displacement of the valve element, and a back pressure chamber communicating with the inlet port through the variable restrictor for biasing the valve element in a valve closing direction. The pilot circuit is connected between the back pressure chamber and the outlet port of the main valve, and is so arranged as to close the main valve when the pilot valve is fully closed and to vary the opening degree of the main valve in accordance with an amount of restriction of the pilot valve. In this connection, the valve apparatus has no means for providing a function equivalent to the above-described counter balance valve.
The invention has been done in view of the actual circumstances of the above-discussed prior art, and it is an object of the invention to provide a valve apparatus capable of securing a function equivalent to a counter balance valve by means of control of a small quantity of flow rate.